System for reading photo-stimulated accumulative luminescent substances

ABSTRACT

An arrangement for reading photo-stimulable storage luminescent substances comprises an excitation glass fiber into which light which excites the storage luminescent substance can be fed by means of a light source. Light emitted by the excited storage luminescent substance can be fed into a receiving glass fiber, an end of the excitation glass fiber which is positioned close to the storage luminescent substance being arranged next to an end of the receiving glass fiber which can be positioned close to the storage luminescent substance. The excitation glass fiber ( 12 ) has a first numerical aperture and the receiving glass fiber has a second numerical aperture which is large compared with the first numerical aperture, as a result of which the light fed out from the excitation glass fiber is directed straight onto the storage luminescent substance without an optical arrangement and the stimulated light can be captured by the receiving glass fiber without an optical arrangement. A glass fiber drum scanner uses the arrangement for reading photo-stimulable storage luminescent substances to achieve a two-dimensional readout of the storage luminescent substance by means of an advancing device which moves the one ends of the excitation glass fiber and the receiving glass fiber in one direction and by means of a rotary drum to which the storage luminescent substance is attached.

FIELD OF THE INVENTION

The present invention relates generally to optical scanners and inparticular to a device for reading photo-stimulable storage luminescentsubstances.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention uses the effect of photo-stimulated luminescence.Phosphorus is one example of the substances which displayphotoluminescence. A storage luminescent substance, e.g. one whichcontains phosphorus, absorbs radiation energy, which excites electronsinto higher energy states. These higher energy states are unstable, sothat the phosphor electrons fall back into a state with lower energy,the energy difference being emitted as light. The emitted light energytypically has a different wavelength than the radiation energy which hasinduced the photoluminescence.

In the field of dental medicine in particular, in which a plurality ofX-ray pictures is created, there is a great need to replace thetraditional X-ray pictures with new imaging techniques. It is timeconsuming and expensive to first expose and then develop X-ray films,the time needed for exposure and development being in generalconsiderably greater than the time spent in actually looking at theX-ray picture in order to make a diagnostic decision. For this reasonX-ray storage luminescent substance foils have recently been usedinstead of the traditional X-ray films, which have to be exposed andthen developed, e.g. in the field of dental medicine. As has alreadybeen mentioned, such storage luminescent substance foils comprise anstorage photoluminescent substance, e.g. phosphorus, which is capable ofretaining a pattern over several days after it has been exposed.Multiple exposure and a higher dynamic range are also advantageousattributes. Image data obtained e.g. by exposure to radioactivesubstances can be recalled using light of the appropriate wavelength. Inparticular, light of the appropriate wavelength stimulates or excitesphotoluminescence in the storage luminescent substance, the lightemitted by the excited storage luminescent substance typically having adifferent wavelength than that of the exciting light.

A glass fibre scanner for scanning an storage phosphor imaging plate isknown from EP 0559118 A1. The glass fibre scanner comprises a forklikeglass fibre bundle with emitter fibres and collecting fibres. Inaddition a device for focusing light into the near end of the emitterfibres is provided, while furthermore a device for focusing light fromthe far end of the emitter fibres onto the phosphor imaging plate and adevice for collecting light at the far end of the collecting fibres areused to increase the sensitivity and wavelength resolution of thearrangement. Such a glass fibre cable is used in the x-y scanner to scanan storage phosphor imaging plate so as to transport light from a lightsource to the phosphors in the storage phosphor imaging plate and tocollect phosphorescence induced by the light.

The glass fibre scanner for scanning an storage phosphor imaging plateaccording to EP 0559118 A1 requires a focusing/imaging lens, which isconnected to the far ends of the at least one emitter fibre and the atleast one collecting fibre. This focusing/imaging lens, which has toserve both to focus the exciting light and to collect the light createdby photoluminescence, makes the whole arrangement more expensive, sinceit must not only be fabricated but it must also be positioned andadjusted.

JP-1-185503 discloses a photodetector with a light receiving surfacewhich is contacted directly by a receiving glass fibre with a core and asheath. The materials of the core and the sheath of the glass fibre areso chosen that the refractive indices of the same decrease at differentrates as the wavelength increases, whereby the numerical aperture of thereceiving glass fibre is wavelength dependent and decreases in thedirection of increasing wavelength. As a result the receiving glassfibre provides relatively good transmission conditions for the lightemitted by a phosphor layer whereas it provides relatively poortransmission conditions for the light for exciting the phosphor, thislight having a longer wavelength than the phosphorescent light emittedby the phosphor layer, whereby the glass fibre acts roughly like ahigh-pass filter. Direct connection of the glass fibre with the lightreceiving surface of the photodetector is thus possible.

DE 2363995 C2 discloses a method for creating a radiographic picture anda device for performing this method. Through the agency of a radiationsource light is here projected through an interference filter onto aregion of an excitable medium, whereupon the excitable medium emitslight radiation which is reflected at the interference filter and isprojected through a lens onto an input area of an image intensifiertube.

U.S. Pat. No. 5,557,452 discloses a confocal microscope. Light radiatedfrom one end of a glass fibre is here focused by a microscope objectiveand radiated through a window onto a sample to be investigated, theresult being that the sample fluoresces. Part of the fluorescent lightradiated by the sample passes through the window again and is fed into aglass fibre by the microscope objective to be processed further.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an arrangement forreading photo-stimulable storage luminescent substances and a glassfibre drum scanner which can be manufactured less expensively.

In accordance with a first object of the invention, this object isachieved by an arrangement for reading photo-stimulable storageluminscent substances comprising: an excitation glass fibre into whichlight which excites the storage luminscent substance can be fed by meansof a light source; and a receiving glass fibre into which the lightproduced by an excited storage luminescent substance can be fed, wherinthat end of the excitation glass fibre which is positionable so as to beclose to the storage luminescent substance is arranged next to that endof the receiving glass fibre which is positionable so as to be close tothe storage luminscent substance, wherein the excitation glass fibre hasa first numerical aperture, and wherein the receiving glass fibre has asecond numerical aperture which is large compared with the firstnumerical aperture, whereby the light which is fed out of the excitationglass fibre is directed straight onto the storage luminscent substancewithout an optical arrangement.

In accordance with a second aspect of the present invention, this objectis achieved by a glass fibre drum scanner comprising: a glass fibrearrangement for reading photo-stimulable storage luminescent substancescomprising an excitation glass fibre into which light which excites thestorage luminescent substance can be fed by means of a light source; anda receiving glass fibre into which the light produced by an excitedstorage luminescent substance can be fed, wherein that end of theexcitation glass fibre which is positionable so as to be close to thestorage luminescent substance is arranged next to that end of thereceiving glass fibre which is positionable so as to be close to thestorage luminescent substance, wherein the excitation glass fibre has afirst numerical aperture, and wherein the receiving glass fibre has asecond numerical aperture which is large compared with the firstnumerical aperture, whereby the light which is fed out of the excitationglass fibre is directed straight onto the storage luminescent substancewithout an optical arrangement; and excitation light source whose lightcan be fed into another end of the excitation glass fibre; a receiverunit for receiving light transmitted in the receiving glass fibre; adrum unit rotatable about a first axis and to the curved surface ofwhich the storage luminescent substance can be affixed; an advancingdevice for moving the closely arranged ends along the first axis; and acontrol unit for synchronizing operation of the excitation light source,the receiver unit, the drum unit and the advancing device.

The present invention is based on the finding that it is possible toachieve a more robust and more economical glass fibre drum scanner bydispensing with the optics for focusing the excitation light and forreceiving the excited light so as to achieve a readout with highresolution and high contrast. The optics for focusing and for receivingthe excited light can the more readily be dispensed with if a glassfibre with a small numerical aperture is used for the excitation glassfibre, whereas, by contrast, the receiving glass fibre must be a glassfibre with a large numerical aperture. The numerical aperture is knownto specialists and is equal to the sine of half the solid angle of amaximum radiation cone which can be supplied by a glass fibre or whichcan be fed into a glass fibre. For a graded-index fibre e.g. thenumerical aperture is calculated from the square root of the differencebetween the mean refractive indices of the glass fibre sheath and theglass fibre core. In accordance with the present invention theexcitation fibre therefore consists of a glass fibre with a smallnumerical aperture, whereby the excitation light cone which it produceshas a small solid angle, which means that, for a still moderateseparation of the end of the excitation glass fibre which is positionednear the storage luminescent substance, a sufficient spot size isattainable, which in the final analysis defines the resolution withwhich the previously exposed storage luminescent substance foil can beread. By contrast, the receiving glass fibre, i.e. the glass fibre whichcollects the light emitted due to photoluminescence, must have a largenumerical aperture, greater at least than the numerical aperture of theexcitation glass fibre, in order that as much as possible of the lightreleased due to photoluminescence can be collected. To increase theamount of photoluminescent light which is detected, and thus increasethe sensitivity of a scanner, a multiplicity of receiving glass fibresmay for preference be arranged around the excitation glass fibre, itbeing obvious to persons skilled in the art that the numerical apertureof a receiving glass fibre of the multiplicity of receiving glass fibresmay then be smaller than the numerical aperture of a single receivingglass fibre while still achieving sufficient sensitivity of the storageluminescent substance scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described in moredetail below, making reference to the enclosed drawings in which

FIG. 1 shows an arrangement for reading photo-stimulable storageluminescent substances according to the present invention;

FIG. 2 shows a cross-section along a line A—A in FIG. 1;

FIG. 3 shows a longitudinal section in the region of the line A—A inFIG. 1; and

FIG. 4 shows a glass fibre drum scanner according to the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 shows an arrangement 10 for reading photo-stimulable storageluminescent substances according to the present invention. Thearrangement 10 comprises an excitation glass fibre 12 and a receivingglass fibre 14, one end 12 a and one end 14 a being arranged in closeproximity to each other in a region which can be positioned near aphoto-stimulable storage luminescent substance. In the region of the oneends 12 a and 14 a the excitation glass fibre 12 and the receiving glassfibre 14 thus form a common bundle 16, which is split into the receivingglass fibre 14 and the excitation glass fibre 12 at a crossover section18. The other end 12 b of the excitation glass fibre is positioned neara light source 20 by means of which light can be fed into the excitationglass fibre 12, which has a small numerical aperture, via a suitablelens 22.

The other end 14 b of the receiving glass fibre, which has a largenumerical aperture compared with the numerical aperture of theexcitation glass fibre 12, is coupled to an optical filter 24 forsuppressing the excitation light, the output of the filter beingconnected effectively to an amplifier circuit 26. The numerical apertureof the excitation glass fibre 12 should lie in a range from 0.1 to 0.2and preferably has a value of 0.12. On the other hand the numericalaperture of the receiving glass fibre 14 should lie in a range from 0.4to 0.9 and preferably have a value of 0.5.

A monomode glass fibre is preferably used for the excitation glass fibre12 to transmit light with a wavelength of 633 nm for a photo-stimulableX-ray storage luminescent substance BaFBr: Eu²⁺, the monomode glassfibre here having a diameter of 3.7 μm. The excitation light from thelight source 20 is fed into the monomode glass fibre by means ofsuitable optics, such as e.g. the lens system 22. The light source maye.g. be a laser or a halogen lamp. The intensity of this light can bevaried. At the one end 12 a of the excitation glass fibre 12 theexcitation light emerges from the monomode fibre and excites the storageluminescent substance to photoluminescence.

As has already been mentioned, a glass fibre with the highest possiblenumerical aperture must be used for the receiving glass fibre 14 inorder to achieve the highest possible collection efficiency. Rather thanjust a single receiving glass fibre 14, a bundle with a multiplicity ofreceiving glass fibres 14 is preferably used, these being arrangedaround the excitation glass fibre 12 at the end of the common bundle 16,as is shown in FIG. 2. The optical filter 24 (FIG. 1), which ispreferably situated in front of the input of the amplifier circuit 26,which may be a photomultiplier, a photodiode or a phototransistor,serves to suppress the excitation light reflected at the one end 12 a ofthe excitation glass fibre 12 so as to prevent this light entering theamplifier circuit 26.

FIG. 3 shows a longitudinal section through the common bundle 16 in theregion of the line A—A in FIG. 1, the essentially concentric arrangementof the single excitation glass fibre 12 within the multiplicity ofreceiving glass fibres 14 being shown here too.

The end of the common bundle 16, i.e. the one end 12 a of the excitationglass fibre 12 and the one end 14 a of the receiving glass fibre 14,lies very close to, and is guided over, an storage luminescent substancefoil containing the photo-stimulable storage luminescent substance.Since the excitation glass fibre 12 has a low numerical aperture, thestorage luminescent substance can be scanned with a high resolution, thephoto-stimulated light emitted by the storage luminescent substancebeing collected with good efficiency due to the high numerical apertureof the at least one receiving glass fibre 14, thus ensuring that thearrangement is sufficiently sensitive. With a monomode fibre having adiameter of 3.7 μm and an excitation glass fibre aperture angle of 12°,corresponding to a numerical aperture of about 0.2, and at a separationbetween the one ends 12 a, 14 a and the storage luminescent substancefoil of 200 μm, an excitation spot of about 45 μm is obtained,corresponding to a resolution of about 11 light points (LP) per mm. Thesize of this spot, or the resolution, thus depends on the numericalaperture of the excitation glass fibre 12 on the one hand and on theseparation between the one end 12 a of the excitation glass fibre andthe storage luminescent substance on the other.

FIG. 4 shows a preferred embodiment of a glass fibre drum scanneraccording to the present invention. The components of the glass fibredrum scanner of FIG. 4 which have already been referred to in FIG. 1 aredenoted by the same reference numerals and will not be described againhereinafter. The glass fibre drum scanner according to the presentinvention comprises a rotatable drum 32 driven by a motor 30, it beingpossible to attach an storage luminescent substance foil to the drum 32.The motor 30 is controlled by a control unit 36, which may e.g. bedirected by a personal computer 38 via suitable interfaces. The motor 30rotates the cylindrical drum 32 about a first axis, as indicated by therotary arrow 40.

The one end 12 a of the excitation glass fibre 12 and the other end 14 aof the at least one receiving glass fibre 14, i.e. the end of the commonbundle 16 which is oriented towards the surface of the cylindrical drum32, are passed through an advancing device 42. Advancing preferablytakes place along the first axis, as indicated symbolically by theadvancing arrow 44. The advancing device 42 is thus able to move thecommon bundle 16 backwards and forwards over the storage luminescentsubstance foil 34 after the motor 30 has rotated the drum 32 so that thestorage luminescent substance foil 34 of FIG. 4 is positioned under theone ends 12 a and 14 a.

The glass fibre drum scanner according to the present invention, whichis shown in FIG. 4, preferably comprises a positioning unit 46 forsetting the separation of the end of the common bundle 16 from thesurface of the drum 32, i.e. from the storage luminescent substance foil34. The positioning unit 46 therefore moves the common bundle along asecond axis which is perpendicular to the first axis, as is shown by thepositioning arrow 48. The separation between the one end 12 a of theexcitation glass fibre 12 and the storage luminescent substance foil 34determines the resolution with which the storage luminescent substancefoil can be read, as has already been explained above.

Various control lines 50 enable synchronous control of the glass fibredrum scanner by the control unit 36, which makes it possible to achievecoordinated operation of the motor 30, the receiver 26, the light source20, the advancing device 42 and the positioning unit 46. The controlunit in turn is connected via a main control line 52 to a computer, e.g.a personal computer or a workstation.

In a preferred embodiment of the glass fibre drum scanner the motor 30rotates the drum 32 at a constant speed. At the same time the advancingdevice 42 moves the common bundle 16 over the storage luminescentsubstance foil 34, thus achieving a two-dimensional raster scan of thesame. The common bundle 16 is thus only moved along the first axis, inconformity with the resolution which can be set by the positioning unit46, since scanning along the second axis is effected by the rotatingdrum 32.

The amplifier circuit 26 is preferably triggered in such a way that animage of the storage luminescent substance foil 34 can be recorded. Inparticular, only those regions of the drum which have previously beenset in a program are regarded as valid image data. As is clear from FIG.4, the whole drum surface does not have to be covered with an storageluminescent substance foil. Storage luminescent substance foils 34 ofarbitrary size can be attached to the drum 32. Several foils can also beapplied simultaneously. The excitation light of the light source 20 canbe controlled in such a way that the light source 20 is switched on whenthere is a valid data region in front of the one end 12 a of theexcitation glass fibre. The excitation light will preferably be switchedoff when there is no valid data region in front of the one end 12 a.This prevents excitation of the foil by scattered light when the foilfinds itself on the rear side of the drum 32 and/or when the one end 12a of the excitation glass fibre 12 is being moved into the readposition.

What is claimed is:
 1. A glass fibre drum scanner comprising: a glassfibre arrangement for reading photo-stimulable storage luminescentsubstance, comprising: an excitation glass fibre for carrying light forexciting the storage luminescent substance; and a receiving glass fibrefor carrying light produced by an excited storage luminescent substance,wherein an end of the excitation glass fibre which is positioned so asto be close to the storage luminescent substance is arranged next to anend of the receiving glass fibre which is positioned so as to be closeto the storage luminescent substance, wherein the excitation glass fibrehas a first numerical aperture, wherein the receiving glass fibre has asecond numerical aperture which is large compared with the firstnumerical aperture, and wherein the end of the excitation glass fibre ispositioned at a predetermined distance to the photo-stimulable storageluminescent substance such that the predetermined distance and the firstnumerical aperture solely define a spot of exciting light having a spotsize defining a resolution for reading information from thephoto-stimulable storage luminescent substance; an excitation lightsource which feeds light into another end of the excitation glass fibre;a receiver unit for receiving light transmitted in the receiving glassfibre; a drum unit rotatable about a first axis and to a curved surfaceof which the storage luminescent substance is affixed; an advancingdevice for moving the closely arranged ends of the glass fibrearrangement along the first axis, and relative to the drum unit; and acontrol unit for synchronizing operation of the excitation light source,the receiver unit, the drum unit and the advancing device.
 2. A glassfibre drum scanner according to claim 1, which furthermore has apositioning unit for positioning the closely arranged ends of theexcitation glass fibre and the receiving glass fibre along a secondaxis, which is perpendicular to the first axis, so as to set aseparation of the one ends from the curved surface of the drum.
 3. Aglass fibre drum scanner according to claim 1, in which the excitationlight source is triggerable by the control unit on the basis ofpreviously set data in order that the excitation light source only emitslight when a region of the storage luminescent substance which isidentified by the preset data is arranged near the one ends of theexcitation glass fibre and the receiving glass fibre.
 4. A glass fibredrum scanner according to claim 1, in which light produced by theexcited storage luminescent substance is collected by the receivingglass fibre, the collection ability of the receiving glass fibre beingdetermined by the second numerical aperture.